The primary aim of this project is to determine whether lymphohematopoietic chimerism can be created by in utero transplantation of allogeneic stem cells. We have used fetal baboons in the past and now are using fetal macaques owing to the greater availability of this species through the Primate Center's timed mating program. After confirmation of gestation age, fetal sex is determined by amniocentesis. If the fetus is female, the in utero transplant is done with allogeneic bone marrow from male donors. The donor cells are enriched for CD34+ cells. The rate of chimerism is evaluated by polymerase chain reaction for Y-chromosome-specific antigens. All transplant procedures have been successful and none of the animals lost as a result of the procedures or protocol. Two animals have shown high degrees of chimerism (6.5% and 10%). Our preliminary data suggest that both HLA and cell dose are important determinants of successful in utero stem transplantation.